A SLM is an optical device for modulating an amplitude, phase or polarization of a light wave in space and in time. The SLM has a number of applications in different fields, e.g., data storage using a holographic method to encode information into a laser beam, realization of a Wavelength Select Switch (WSS) and use in Photolithography.
One practical realization of the SLM is based on the liquid crystal on silicon (LCOS) technology. In the SLM, a liquid crystal (LC) layer is positioned between a transparent electrode layer and a reflecting electrode layer, where the reflecting electrode comprises an array of pixel electrodes and is built on a silicon substrate. When a voltage difference is applied between the transparent electrode layer and one pixel electrode, LC molecules therebetween are re-orientated with an applied electric field. Since the LC is birefringent, i.e. having a refractive index that depends on the polarization and propagation direction of light in the LC, the orientation results in a phase shift, commonly known as a phase retardation, to the light where the phase retardation is controllable by the voltage difference due to the Electric Controlled Birefringence Effect, (ECB Mode).
One undesirable factor that leads to uncertainty in the amount of phase retardation that is produced is a FFE. The FFE is that the electric field generated at the boundary of a pixel electrode leaks to a neighboring pixel, affecting the LC alignment at the neighboring pixel and thereby generating unwanted phase shift to the light incident on the neighboring pixel. The unwanted phase shift is different at different places on the neighboring pixel, and is most pronounced around the boundary of the neighboring pixel. As the presence of FFE can significantly deteriorate the SLM performance, such as a considerable reduction of diffraction efficiency and phase profile accuracy, it is advantageous if the FFE can be substantially reduced.
In the art, there are several techniques for reducing the FFE. CN103645591 teaches inserting an additional electrode between two adjacent pixel electrodes so as to shield the electric field generated by one pixel from influencing another pixel. However, non-uniformity in the resultant electric field impacts the LC orientation, causing a phase curve error. In US20070052889, corners of a pixel electrode are rounded in order to reduce the electric fields generated at the corners. Instead of using a rectangular pixel electrode, US20150002795 teaches using a non-rectangular pixel electrode to compensate for the FFE by changing the electric field around the edge of the pixel electrode. While the last two techniques can reduce the FFE to a certain extent, the reduction may not be sufficient for certain practical applications as the extent of structural change made to the pixel electrode is constrained by the size thereof and there is a trend of shrinking the electrode size. There remains a need in the art for an improved technique to reduce the FFE.